<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
</head>
<body>
  
  <script>
    var year = parseInt( prompt('请输入年份') )  // 强制转换 整型数字

    var age = 2021 - year  // 隐式类型转换

    alert('今年' + age + '岁了') // 引引加加
  </script>
</body>
</html>